1. Field of the Invention
The present invention relates to a magneto-optical recording medium, and more specifically, it relates to a magneto-optical recording medium comprising a recording layer for recording a signal and a reproducing layer for reproducing the signal after transferring a magnetic domain in the recording layer.
2. Description of the Background Art
A magneto-optical recording medium, which is noted as a reloadable and highly reliable recording medium having a large storage capacity, is put into practice as a computer memory or the like, to come into wide use. Following increase in information content and downsizing of devices, a recording/reproducing technique of higher density is required.
A high-density recording/reproducing technique includes that for a device and that for a medium. The technique for the device includes optical super-resolution for obtaining a condensed spot beyond the diffraction limit for a laser beam, reduction of the wavelength of the laser beam and the like. The technique for the medium includes reduction of the pitch of the medium, improvement of reproducing resolution with a magnetic multilayer film and the like (refer to Y. Murakami et al. xe2x80x9cSuper Resolution Readout of a Magneto-Optical Disk with an In-Plane Magnetization Layerxe2x80x9d, Proceedings of Magneto-Optical Recording International Symposium ""92, J. Magn. Soc. Jpn. Vol. 17, Supplement No. S1 (1993), pp. 201 to 204). The technique of improving the reproducing resolution with the magnetic multilayer film is adapted to selectively transfer a magnetic domain of a recording layer into a reproducing layer for reproducing the same through the temperature distribution of a laser spot forming Gaussian distribution around the center
FIGS. 1A and 1B show the signal recording principle in a conventional magneto-optical recording medium having a recording layer 6 and a reproducing layer 3 which are in contact with each other. In order to record a signal in such a medium, a magnetic field Hx is applied from the recording layer 6 side while a laser beam LB is simultaneously applied from the reproducing layer 3 side. In general, the Curie temperature of the recording layer 6 is lower than that of the reproducing layer 3. When the signal is recorded, therefore, the temperature of the medium rises to a level between the Curie temperatures of the recording layer 6 and the reproducing layer 3 or higher than the Curie temperature of the reproducing layer 3. When the magnetic field Hx is applied from the exterior in such a state, a magnetic domain 30 magnetized in the same direction as the magnetic field Hx is formed in the reproducing layer 3 as shown in FIG. 1A. When the temperature of the region formed with the magnetic domain 30 is thereafter reduced below the Curie temperature of the recording layer 6, the magnetic domain 30 in the reproducing layer 3 is transferred into the recording layer 6 by exchange interaction, to define a magnetic domain 60 in the recording layer 6 as shown in FIG. 1B.
In order to record the signal in such a conventional medium, therefore, recording conditions must be decided in consideration of the characteristics of the recording layer 6 and the reproducing layer 3. The signal is recorded by exchange interaction from the reproducing layer 3 to the recording layer 6, whereby the magnetic properties of the reproducing layer 3 exert influence on the signal recording. While the magnetic influence from the reproducing layer 3 to the recording layer 6 can be reduced by increasing the temperature of the reproducing layer 3 beyond the Curie temperature, higher laser power is required in this case since the Curie temperature of the reproducing layer 3 is higher than that of the recording layer 6. In order to transfer the magnetic domain 30 in the reproducing layer 3 into the recording layer 6 by exchange interaction, further, the Curie temperature of the reproducing layer 3 must be increased beyond that of the recording layer 6.
In order to reproduce the signal from such a conventional medium, on the other hand, the transition temperature at which the reproducing layer 3 changes from an in-plane magnetization film to a perpendicular magnetization film must be increased since the magnetic domain 60 in the recording layer 6 is selectively transferred into the reproducing layer 3 by exchange interaction. Thus, the laser power must be increased, and the temperature of the recording layer 6 so rises that selectivity for the magnetic domain 60 transferred from the recording layer 6 into the reproducing layer 3 is reduced as a result.
An object of the present invention is to provide a magneto-optical recording medium which can stably record a signal in a recording layer with no influence by magnetization of a reproducing layer.
Another object of the present invention is to provide a magneto-optical recording medium which can record a signal with low laser power.
Still another object of the present invention is to provide a magneto-optical recording medium which can record a signal in higher density.
A further object of the present invention is to provide a magneto-optical recording medium having high selectivity for a magnetic domain transferred from a recording layer into a reproducing layer.
The magneto-optical recording medium according to the present invention comprises a first reproducing layer, a magnetic shielding layer and a first recording layer. The first reproducing layer changes from an in-plane magnetization film to a perpendicular magnetization film at a first transition temperature. The magnetic shielding layer is formed on the first reproducing layer. The first recording layer is formed on the magnetic shielding layer, and saturation magnetization thereof is maximized around the first transition temperature.
Preferably, the magneto-optical recording medium further comprises a second reproducing layer. The second reproducing layer is formed on the first reproducing layer oppositely to the magnetic shielding layer, and changes from an in-plane magnetization film to a perpendicular magnetization film at a second transition temperature which is higher than the first transition temperature.
Preferably, the magneto-optical recording medium further comprises a second recording layer which is formed on the first recording layer in a compensating composition.
Preferably, the first recording layer includes a plurality of magnetic films and a plurality of non-magnetic films which are formed alternately with the plurality of magnetic films.
In the aforementioned magneto-optical recording medium, the magnetic shielding layer is formed between the first recording layer and the first reproducing layer, whereby a signal is directly recorded in the first recording layer. Thus, the first recording layer is not magnetically influenced by the first reproducing layer, whereby the signal can be correctly recorded with low laser power. On the other hand, the magnetic domain formed in the first recording layer is transferred into the first reproducing layer by static magnetic interaction at the first transition temperature, and further transferred into the reproducing layer by exchange interaction at the second transition temperature. Thus, the magnetic domain is transferred from the first recording layer into the first reproducing layer at a temperature lower than that in the prior art, whereby the laser power for reproduction can be reduced to improve selectivity of the magnetic domain from the first recording layer to the first reproducing layer.
The second reproducing layer is further formed on the first reproducing layer, whereby the magnetic domain transferred into the first reproducing layer is further transferred into the second reproducing layer. Therefore, the selectivity for the magnetic domain from the first recording layer to the reproducing layer is further improved so that the signal can be reproduced in higher density.
The second recording layer is formed on the first recording layer in the compensating composition, whereby the signal can be further stably recorded in the first recording layer.
The plurality of magnetic films and the plurality of non-magnetic films are alternately formed on the first recording layer, whereby the non-magnetic films prevent thermal diffusion in the first recording layer and the laser power can be further reduced.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.